It is generally recognized that in the future an increasing percentage of television broadcasting plant processing techniques will be performed digitally. At present, color encoding and decoding in the digital domain are relatively costly operations because arithmetic operations must be performed at high speeds that tax the capabilities of existing logic hardware. However, as digital circuits and stores become faster and less expensive, the concepts of digital processing become increasingly practical.
An advantageous type of digital encoding for a color television signal employs a pulse-code modulation (PCM) encoding technique. In PCM, the modulating signal waveform is sampled at regular intervals and the samples are quantized into discrete steps. Only certain discrete levels are allowed within a specified range of expected sample values and these are represented in the system by means of a code pattern of a series of pulses that are typically in binary code. The number of allowed discrete levels is determined by the word length; e.g., 6 binary digits or bits permits 64 levels, 8 bits permits 256 levels, etc., so it is evident that both the resolution and the complexity of the system largely depend on the word length chosen. A PCM signal has the advantage of high immunity to the effects of channel noise and channel amplitude non-linearity.
Once a color television signal has been converted into digital form, such as by a PCM technique, it may be transmitted in this form and then converted back into an analog signal which is, in turn, decoded to obtain the baseband signals, I, Y and Q. It is envisioned, however, that there will be substantial advantage if one could readily decode a pulse-code modulated NTSC color television signal into digital versions of the I, Y, and Q baseband signals without first converting into the analog domain.
In color decoding a PCM NTSC signal, it is desirable that the digital sampling rate be a multiple of the sub-carrier frequency, so that during sampling the phase relationship between the PCM sampling signal and the NTSC subcarrier would be known and consistent. This being the case, and considering the minimum constraints imposed by sampling theory, the most likely choices for a suitable sampling frequency are nominally 10.74 MHz. (precisely three times the color subcarrier frequency) or nominally 14.32 MHz (precisely four times the color subcarrier frequency). As between these choices, the 10.74 MHz. sampling frequency has the obvious advantage of requiring lower speed circuitry and lower storage requirements than the 14.32 MHz. sampling frequency. However, as will be demonstrated, there is an unfortunate problem associated with attempting to decode a digital NTSC color television signal that has been sampled at 10.74 MHz.
It is a desirable technique of television processing to separate the luminance and chrominance components of a composite NTSC signal by "comb filtering". A comb filter takes advantage of the frequency relationship between the horizontal line rate and the color subcarrier signal. As is well known, a typical comb filter utilizes three adjacent television lines in a given field and selectively adds and subtracts them to obtain the chrominance and luminance signals. Specifically, the typical analog comb filter combines three adjacent lines of a given field designated top (T), middle (M), and bottom (B), in the following proportions to obtain the chrominance (C) and the luminance (Y) signals: EQU C = M - (1/2) (T + B) EQU y = m + (1/2) (t + b)
an examination of the function of an analog comb filter reveals that it effectively operates by sampling and averaging, with particular weighting coefficients, three picture elements from three adjacent lines. This sampling and averaging is repeated for all individual picture elements (a picture element being considered as an infinitesmal image sample). Thus, a digital PCM television signal would appear ideal for comb filtering since each digital code word describes the instantaneous amplitude of the analog signal at a particular sampling time. This is indeed the case for a PCM signal obtained by sampling at 14.32 MHz. (or an even multiple of the color subcarrier) since video samples on successive lines are separated by exactly one horizontal scan period. However, when the television signal is encoded at an odd multiple of the color subcarrier frequency, 10.74 MHz. for example, the digital samples on sequential lines are found to be vertically misaligned. As will be shown, this result follows from the odd multiple relationship between one-half the line rate and the color subcarrier frequency in the NTSC system. The misaligned samples render difficult the the comb filtering of a digital PCM TV signal that had been sampled at 10.74 MHz. Since 14.32 MHz. is the lowest usable even multiple that could be used as a sampling frequency, and this higher frequency is disadvantageous from the standpoint of storage size and circuit speed it would be desirable if the problem of comb filtering a digital television signal sampled at 10.74 MHz. could be solved. It is an object of the present invention to provide this solution.